Air or oxygen depolarization cells are well known in the art. Electrochemical cells of this type include a gas diffusion cathode capable of generating electricity. Generally, these electrochemical cells are comprised of spaced apart electrodes ionically connected by an electrolyte.
Until recently, metal/air cells were not suitable for applications in which a rapid discharge was required. Presently, however, metal/air cells are capable of achieving high energy to density ratios, making them useful in rapid discharge applications such as light weight intra-urban vehicles.
The air electrodes utilized in present state-of-the-art metal/air cells, however, are generally unifunctional; that is, they are incapable of electrochemically regenerating the fuel electrode. Therefore, in order to recharge the battery, a third, counter-electrode is required. Since the counter-electrode is used only during the recharging cycle and the air electrode only during discharge, the cell requires a complicated geometric design, as well as complex mechanical or electrical switching means. Additionally, the use of a counter-electrode increases the dead weight of the cell, and reduces the cell voltage, resulting in a deterioration of the capacity of the metal electrode on extended cycling.
Accordingly, it is an object of the present invention to provide an improved electrode for use in hydrogen/oxygen fuel cells, and particularly, a bifunctional air electrode for use in metal/air cells that undergo substantial charge-discharge cycling. It is a further object of the invention to provide, as the preferred embodiment a bifunctional air electrode which not only eliminates the need for a counter-electrode, but which has a long cycling life, stability, high energy capacity, and low cost.
From an analysis of the reactions associated with oxygen reduction and evolution, four different categories of active components are necessary in a useful bifunctional air electrode: (1) a conductive support; (2) an oxygen absorption/reduction electrocatalyst, generally a carbon material; (3) a catalyst for perhydroxide ion decomposition and (4) an electrocatalyst for oxygen evolution. Ideally one or several materials could perform all of these functions.
Presently, the carbon used as the oxygen absorption/reduction electrocatalyst is the weak component in bifunctional air electrodes. At the high potentials associated with oxygen evolution, the active sites of the carbon are unstable. This instability is considered to be a major contributor to the observed performance degradation of carbon type bifunctional air electrodes on cycling. In order to provide improved bifunctional air electrodes, low cost substitute oxygen absorption/reduction electrocatalysts must be found, preferably one which could also provide other catalytic functions.
The ideal oxygen absorption/reduction electrocatalyst for use in the air electrode of a metal/air battery will have the following properties:
1. Stability between +750 mV and -300 mV (relative to Hg/HgO) in 25 wt.% KOH in the temperature range 0.degree.-70.degree.C.
2. Electrical resistivity of less than 1000 ohm-cm.
3. Negligible contact resistance between the conductive support and the electrocatalyst in 25 wt.% KOH.
4. Capability of delivering 50 mA cm.sup.-.sup.2 at -150 mV (relative to Hg/HgO) in a fully optimized electrode structure for 1000 charge-discharge cycles.
The current price of gold and platinum, which are active in the pure or alloyed state, rule out their large scale use in commercial metal/air batteries. Strontium-doped lanthanum cobaltite LaCoO.sub.3 :Sr was the first material to indicate the feasibility of economical substitution for carbon in air electrodes. This material however does not fulfill condition 3 above, in that activity is present in this material only in sintered plaque form. Other perovskites, such as LaMnO.sub.3 :Sr and LaNiO.sub.3 :Sr have proved to be ineffective. The oxygen/reduction properties of spinels, for example, CoFe.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, NiCo.sub.2 O.sub.4 and NiFe.sub.2 O.sub.4, does not suggest that these compounds are strong candidates for use in the air electrodes of metal/air batteries. What is needed then is a new, highly-active, effective electrocatalyst, preferably one that will be inexpensive.